1. Field of the Invention
The present invention relates to a zoom lens incorporated in a television camera or video camera, for which high resolution, greater variable power, wider angle of view, smaller size, lighter weight, and the like are required and, in particular, to a zoom lens which exhibits a favorable optical performance in the whole variable power range while satisfactorily yielding a great variable power, a wide angle of view, a smaller size, and a lighter weight.
2. Description of the Prior Art
Zoom lenses for television camera, video camera, and the like have conventionally been required to have a great variable power and a high performance.
Almost all the imaging devices used in the current cameras are CCDs (solid-state imaging devices), for which a high resolution is required in the whole screen area. Consequently, it is necessary for a zoom lens used in such a camera to uniformly have a high resolution even in peripheral portions of the screen in the whole variable power range.
Also, as the camera becomes smaller, the zoom lens used therefor is required to have a smaller size as well.
Further, in general, a long back focus is needed in a television camera lens for broadcasting station since a color-separating optical system and various kinds of filters are inserted in front of its imaging device.
Known as a zoom lens satisfying such a requirement are a number of configurations each comprising a first group, fixed upon varying power, having a positive refracting power; a second group having a power-varying effect and a negative refracting power; a third group having a positive refracting power for correcting an image surface shift generated upon the power-varying effect of the second group; and a fourth group, fixed upon varying power, having an imaging effect.
For example, the one disclosed in Japanese Unexamined Patent Publication No. 6-59191 has a lens configuration in which each group has an appropriate allocation of refracting power, while a suitable lens surface is shaped into an aspheric form, in order to attain a high optical performance throughout the variable power range.
In the above-mentioned television zoom lens for broadcasting station, the focal length of the power-varying portion, i.e., that of each of the second and third groups, may be made as short as possible so that the length of the power-varying portion at its wide angle end is minimized, and together therewith, the outer diameter of the first group may be made smaller in order to reduce the size and weight of the whole lens. In this case, however, since the negative refracting power of the second group increases, Petzval sum tends to become greatly negative.
It has conventionally been known that Petzval sum is required to be made lower in order to reduce astigmatism and attain a flatter image surface (disclosed, for example, in Yoshiya Matsui, Renzu Sekkei Hou (Lens Designing Method), Kyoritsu Shuppan Co., pp. 110-111). In particular, in the television zoom lens requiring a high resolution, the sagittal image surface in the peripheral portion of the screen may especially deteriorate in the whole variable power range unless the Petzval sum value is appropriately set.